Hockey Stick Controversy

The hockey stick controversy refers to debates over the technical correctness and implications for global warming of graphs showing reconstructed estimates of the temperature record of the past 1000 years. At a political level, the debate is about the use of these graphs to convey complex science to the public, and the question of the robustness of the assessment presented by the Intergovernmental Panel on Climate Change (IPCC). Arguments over the graphs have been taken up by fossil fuel industry funded lobbying groups attempting to cast doubt on climate science.

By the late 1990s a number of competing teams of climate scientists were using proxy indicators to estimate the temperature record of past centuries, and finding suggestions that recent warming was exceptional. In 1998 Michael E. Mann, Raymond S. Bradley and Malcolm K. Hughes used statistical analysis of a variety of proxies to produce the first quantitative hemispheric-scale reconstruction showing global patterns of annual surface temperature. It included a graph going back to 1400, with the innovation of shading emphasising that uncertainties (to two standard error limits) were much greater in earlier centuries. Their 1999 paper using this methodology went back to 1000, with the extended graph showing a downward trend in proxy temperatures from a Medieval Warm Period followed by a 20th century rise and a steep increase in measured temperatures since the 1950s. The term hockey stick was coined by the climatologist Jerry Mahlman, to describe the pattern this showed, envisaging a graph that is relatively flat to 1900 as forming the hockey stick's "shaft", followed by a sharp increase corresponding to the "blade".

A version of this graph was featured prominently in the 2001 United Nations Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR), which also drew on four other reconstructions to support the conclusion that, in the Northern Hemisphere, the 1990s was likely to have been the warmest decade and 1998 the warmest year during the past 1,000 years. The graph was featured in publicity, and became a focus of dispute for those opposed to the strengthening scientific consensus that late 20th century warmth was exceptional.

In 2003, Willie Soon and Sallie Baliunas argued against this pattern in a paper which was dismissed by many scientists as deeply flawed in the Soon and Baliunas controversy. In the United States there was already a hot political dispute over action on global warming following lobbying regarding the 1997 Kyoto Protocol, and on July 28, Republican Jim Inhofe made a Senate speech citing Soon and Baliunas to support his belief "that man-made global warming is the greatest hoax ever perpetrated on the American people". Later in 2003, Stephen McIntyre and Ross McKitrick published a paper questioning the data used in the Mann, Bradley and Hughes 1998 paper, and saying they could not get the statistical methods to produce the same results. In 2004 Hans von Storch published criticism of the statistical techniques as tending to underplay variations in earlier parts of the graph, though this was disputed and he later accepted that the effect was very small. In 2005 McIntyre and McKitrick published criticisms of the principal components analysis methodology as used in the 1998 and 1999 papers, though not used in subsequent papers. Their analysis in turn was disputed by published papers including Huybers 2005 and Wahl & Ammann 2007 which pointed to errors in the McIntyre and McKitrick methodology. At the request of Congress, a panel of scientists convened by the National Research Council was set up, which reported in 2006 supporting Mann's findings with some qualifications, including agreeing that there were some statistical failings but these had little effect on the result. U.S. Rep. Joe Barton and U.S. Rep. Ed Whitfield requested Edward Wegman to set up a team of statisticians to investigate, and they supported McIntyre and McKitrick's view that there were statistical failings, although they did not quantify whether there was any significant effect. They also produced an extensive network analysis which has been discredited by expert opinion and found to have issues of plagiarism.

The test in science is whether findings can be replicated using different data and methods. Numerous scientific papers, using various statistical methods and combinations of proxy records, produced reconstructions broadly similar to the original 1998 hockey-stick graph, with variations in how flat the pre-20th century "shaft" appears. The 2007 IPCC Fourth Assessment Report cited 14 reconstructions, 10 of which covered 1,000 years or longer, to support its strengthened conclusion that it was likely that Northern Hemisphere temperatures during the 20th century were the highest in at least the past 1,300 years. Eight or more subsequent reconstructions, including Mann et al. 2008, have supported these general conclusions.